The present invention relates to a base station and a scheduling method thereof, and more particularly to a base station which transmits an identical packet or data to mobile stations using a plurality of transmission antennas based on a transmission diversity system, and a scheduling method thereof.
The transmission diversity technology (Tx-Div technology) is a downlink communication technology from a base station to terminals (mobile stations) in a WCDMA system, and is used to decrease reception interference in mobile stations. According to this transmission diversity technology an identical packet or data is transmitted to mobile stations via a plurality of transmission antennas. HSDPA (High Speed Downlink Packet Access) technology is a high speed communication method to transmit packets from a base station of a WCDMA system to mobile stations.
(1) Transmission Diversity
Closed Loop Transmission Diversity System
FIG. 13 is a diagram depicting a closed loop transmission diversity system among transmission diversity technologies. In a closed loop transmission diversity system, a plurality of antenna elements are disposed in a wireless base station in a cellular mobile communication system, and (1) a different amplitude and phase control is performed for an identical transmission data signal based on feedback information FBI which is sent from a mobile station, thereby transmission data is generated for each antenna element, (2) a pilot signal is multiplexed with each transmission data which is obtained by the different amplitude and phase control, and multiplexed results are transmitted via a plurality of antennas, and (3) the mobile station side receives the transmission data, and decides the feedback information that instructs amplitude and phase control amount using the downlink pilot signal, multiplexes this feedback information with an uplink channel signal, and transmits it to the base station side, and the above operation is repeated hereafter.
In the case of the closed loop transmission diversity in W-CDMA, that is the third generation mobile communication system, a system using two transmission antennas, shown in FIG. 13, is adopted. In FIG. 13, pilot patterns P1 and P2, which are orthogonal to each other, are generated in a pilot signal generation unit 11, are combined with transmission data in combining units CB1 and CB2, and are transmitted via transmission antennas 10-1 and 10-2 respectively. A channel estimation unit (not illustrated) at the reception side of the mobile station computes the correlation of the received pilot signal and the known pilot pattern, so as to estimate the channel impulse response vectors h1 and h2 from each transmission antenna 10-1 and 10-2 of the base station to the mobile station reception antenna 12.
A weight calculation unit 13 calculates an amplitude and phase control vector (referred to as weight vector) w=[w1, w2]T of each transmission antenna 10-1 and 10-2 of the base station, so that the power P given by the following Expression (1) becomes the maximum, using these channel estimation values h1 and h2. Then this weight vector is quantized and multiplexed with the uplink channel signal as the feedback information, and is transmitted to the base station. It is not necessary to transmit both values w1 and w2, but is sufficient to transmit only value w2, which is determined with w1=1.P=wHHHw   (1)H=[h1, h2]  (2)Here h1 and h2 are channel impulse response vectors from the antenna 10-1 and antenna 10-2 to the mobile station reception antenna 12 respectively. The HH and wH denotes the Hermite conjugates of the matrices H and w.
In a mobile station, the weight coefficient unit 13 calculates the weight factor (weight vector), as mentioned above, and a multiplexing unit 18 multiplexes this weight factor with the uplink transmission data as the feedback information FBI, and sends it to the base station via a transmission antenna 14.
In the base station, the feedback information from the mobile station is received via a reception antenna 15, a feedback information extraction unit 16 extracts the weight factors w1 and w2, which are control amounts, and an amplitude/phase control unit 17 multiplies the downlink transmission data by the weight factors w1 and w2 respectively using multiplexers MP1 and MP2, so as to control the amplitude and phase of the signals to be transmitted via the transmission antennas 10-1 and 10-2. Thereby the mobile station can efficiently receive the signals which are transmitted from the two diversity transmission antennas 10-1 and 10-2. Ideally it is preferable that the signals, which are transmitted from the two diversity transmission antennas 10-1 an d 10-2, reach and are received by the reception antennas 12 in a state of having a same phase.
Feedback Information FBI
In W-CDMA, two methods, that is mode 1 which quantizes the weight factor w2 into 1 bit, and mode 2 which quantizes the weight factor w2 into 4 bits, are specified. Mode 1 is a method for controlling the phase of a reception signal from each transmission antenna to be roughly a same phase with a π/4 resolution, where 1-bit feedback information is transmitted via each slot, and controlled. Mode 2 is a method for controlling the phase of a reception signal from each transmission antenna to be roughly a same phase with a π/4 resolution, and also controls the ratio of transmission power of the transmission signal from each transmission antenna using 4-bit information.
FIG. 14 is a diagram depicting the configuration of an uplink DPCH (Dedicated Physical Channel)frame, standardized according to the 3rd Generation Partnership Physical projects (hereinafter referred to as 3GPP), where DPDCH (Dedicated Physical Data Channel) over which only transmission data is sent, and DPCCH (Dedicated Physical Control Channel) over which such control data as pilot and feedback information is sent, are multiplexed by orthogonal codes. In a frame format of an uplink signal from the mobile station to the base station, one frame is 10 msec., and consists of 15 slots (slot #0 to slot #14). DPDCH is mapped on an orthogonal I channel in QPSK modulation, and DPCCH is mapped on an orthogonal Q channel in QPSK modulation. Each slot of DPDCH consists of n (=N data) bits, and n changes according to the symbol speed. Each slot of DPCCH consists of 10 bits, the symbol speed is constant at 15 ksps, and transmits pilot PILOT, transmission power control data TPC, transport format combination indicator TFCI, and feedback information FBI. PILOT is used by the reception side for channel estimation (estimating propagation path characteristics) and measuring Signal to Interface Ratio (SIR), TPC is used for transmission power control, and TFCI is used for transmitting a symbol speed of data and the number of bits per frame, or the like, and FBI are used for transmitting the above mentioned feedback information (weight factor) for controlling the transmission diversity in the base station.
Based on the transmission power control bit TPC, the base station controls transmission power to the mobile station, so that the reception Signal to Interface Ratio (SIR) of the mobile station becomes constant.
Configuration of Wireless Mobile Station
FIG. 15 is a configuration example of the wireless mobile station, where a downlink data signal from the base station is received by the reception antenna 12, divided into data and pilot, and sent to a data channel inverse spread unit 20 and a pilot channel inverse spread unit 22 respectively. The data channel is inverse-spread in the data channel inverse-diffusion unit 20, and the pilot channel is inverse-spread in the pilot channel inverse-spread unit 22. The pilot signals P1′ and P2′ which are processing results of the pilot channel inverse spread unit 22, are input to channel estimation units 23-1 and 23-2 and the weight calculation unit 13.
The channel estimation units 23-1 and 23-2 compares the received pilot signals P1′ and P2′ and the known pilot signals P1 and P2 in order to determine each channel estimation value from the transmission antennas 10-1 and 10-2 of the base station to the reception antenna 12. Then the channel estimation units 23-1 and 23-2 estimates the channel impulse responses h1 and h2 and input the responses to a reception unit 21. The reception unit 21 performs channel compensation processing on the data channel signals based on the channel impulse responses, and inputs the result to the demodulation and decoding units, which are not illustrated.
The weight calculation unit 13 determines the weight factors w1 and w2 which maximize the power P given by Expression (1), and outputs the feedback information FBI. In other words, an phase/amplitude comparison unit 13a of the weight calculation unit 13 compares the phases and amplitudes of the pilot signals P1′ and P2′ received from the transmission antennas 10-1 to 10-2, and outputs the weight factors w1 and w2, an FBI generation unit 13b generates a feedback FBI corresponding to these weight factors w1 and w2, and inputs them to the multiplexing unit 18, and the multiplexing unit 18 multiplexes the feedback information and the transmission data signal. A data modulation unit 25 performs orthogonal modulation based on the multiplexed data, a spread modulation unit 26 spread and modulates the data, and transmits the uplink data, including the feedback information from the transmission antenna 14, to the base station.
(2) HSDPA
HSDPA is a high-speed packet communication technology. As FIG. 16 shows, the main wireless channels used for HSDPA are (1) HS-SCCH (High Speed Shared Control Channel), (2) HS-PDSCH (High Speed Physical Downlink Shared Channel), and (3) HS-DPCCH (High Speed Dedicated Physical Control Channel).
Both HS-SCCH and HS-PDSCH are downlink shared channels, of which HS-SCCH is a control channel used to send various parameters on packets which are transmitted via HS-PDSCH.
In other words, HS-SCCH is a channel to notify the mobile station that packets are transmitted via HS-PDSCH. The various parameters include, for example, destination information of a mobile station to which the packet is sent, modulation system information as to which modulation system is used for transmitting the packet via HS-PDSCH, and information on the pattern of rate matching to be performed on the transmission data.
HS-DPCCH, on the other hand, is an uplink dedicated control channel, and is used for transmitting an ACK signal/NACK signal to indicate the presence of an error in the data which the mobile stations 71 and 72 received via HS-PDSCH respectively, and CQI (Channel Quality Indicator) to indicate the reception quality, to the wireless base station. The wireless base station performs retransmission control H-ARQ (Hybrid Automatic Repeat reQuest) based on the ACK signal/NACK signal. In addition, the wireless base station judges the quality of the downlink wireless environment by CQI and switches to a modulation system with which packets are transmitted faster if the environment is good, or switches to a modulation system with which packets can be transmitted slower if not (in other words, adaptive modulation is performed).
Problem the Invention is to Solve
When identical packets are transmitted on a shared channel using HSDPA, via a plurality of transmission antennas based on the transmission diversity system, a problem is that interference to a mobile station which is not using HSDPA, such as a mobile station communicating voice and images via a dedicated channel DPCH, may increase depending on the scheduling of HSDPA.
This is because in HSDPA, a plurality of mobile stations share a channel, and packets are transmitted at high power. In particular, the interference of HSDPA on voice communication increases and communication quality thereof deteriorates if a transmission phase which generates at a time when packets are diversity-transmitted to mobile stations using a shared channel based on HSDPA, accords with a transmission phase which generates at a time when voice data is diversity-transmitted to mobile stations using a dedicated channel.
To prevent this, a scheduler determines a shared channel is allocated to which mobile station, and how long the channel is allocated to the mobile station, considering (1) the environment of each mobile station, and (2) the time the data destined for each mobile station stays in the base station. However, a problem is that current scheduling system is insufficient, and interference of HSDPA on non-HSDPA communication, such as voice communication, is considerable.
A first prior art is a technology for rotating the phase of the transmission diversity antennas based on the FBI information from the mobile stations under HSDPA using transmission diversity (JP2005-260634A). However, this first prior art does not decrease the interference of HSDPA on non-HSDPA communication.
A second prior art is a wireless data transmission system for transmitting data to predetermined mobile terminals from the network side via a shared wireless line (WO2006/095387). In this second prior art, the scheduler calculates an index value to select a mobile terminal based on the quality of a receive signal in each mobile terminal, corrects this index value using a quality fluctuation rate of the reception signal, or a fading frequency or error rate for each CQI, for example, and selects a mobile terminal to which data is transmitted via the shared wireless line, based on this corrected index value. However this second prior art does not decrease the interference of HSDPA on non-HSDPA communication either.